The present invention relates to what is called an ultrasonic switching apparatus for detecting the presence or absence of an object, a distance to the object, the presence or absence of an object in a predetermined range, or the like by use of ultrasonic waves and, more particularly, to an ultrasonic switching apparatus of what is called the amplifier separate type in which an ultrasonic sensor unit and a signal processing unit (also called an amplifier unit) are separated and these units are connected by a cable.
An apparatus to detect an object using ultrasonic waves is generally an apparatus of the reflection type. Namely, pulse-like ultrasonic waves are transmitted and the reflected waves returned by being collided with an object to be detected are received. In principle, when the reflected waves are received, a detection signal of the object is output. When the level of the reflected waves is detected and it is above a predetermined level, the object detection signal is output. There is also an apparatus which determines that an object could be detected only when the reflected waves were received within a constant period of time after the pulse-like ultrasonic waves had been transmitted. In particular, the latter apparatus is used to detect an object which is located within a range of a predetermined distance from the position of the ultrasonic sensor. By measuring the time until the reflected waves are received after the ultrasonic waves had been transmitted, the distance from the position of the ultrasonic sensor to an object can be also measured. The ultrasonic waves are propagated in the air at the sound velocity and the sound velocity changes depending on the temperature. Therefore, in the case of considering a factor of the distance (which is obtained by multiplying the sound velocity with the time) in the detecting or measuring process, the temperature correction needs to be performed, so that it is inevitable to measure the temperature.
In an ultrasonic detecting apparatus of the type such that an ultrasonic sensor unit and a signal processing unit are separated, both units are connected by a cable (lines). This cable needs total six core wires (or cores, lines or conductors) consisting of five core wires to transmit at least the following signals and a OV line to made the grounding levels of both units coincident.
The following signals are transmitted from the signal processing unit to the ultrasonic sensor unit:
(1) a pulse-like transmission voltage signal to pulse-drive an ultrasonic transducer of the ultrasonic sensor unit;
(2) a power supply signal to make each circuit, element, and the like of the sensor unit operative; and
(3) an operation indicating signal to indicate the detection of an object by lighting up an operation indicator provided for the sensor unit.
The following signals are transmitted from the sensor unit to the signal processing unit:
(1) a reception signal which is generated when the reflected waves from the object to be detected were received; and
(2) a temperature detection signal which is output from a temperature sensor provided for the sensor unit and is used to perform the sound velocity correction.
Since the cable having many core wires is necessary as described above, there are problems such that the cost increases by an amount as much as the number of core wires and at the same time, the working efficiency deteriorates in the field where the ultrasonic detecting apparatus is attached.